FIG. 1 depicts a schematic diagram of a portion of wireless local area network 100 in the prior art. Local area network 100 comprises stations 101, 102-1, and 102-2. Stations 101, 102-1, and 102-2 use a shared-communications channel to communicate among themselves and only one of the terminals can transmit into the channel at a time. When two terminals transmit into the channel at the same time, the result is a cacophony and both transmissions are garbled.
Therefore, a technique called Carrier Sense Multiple Access is used by the stations to coordinate when each of them transmits. In accordance with this technique, each radio listens to the shared-communications channel and waits to transmit until the channel is quiet (i.e., no other stations are transmitting). Carrier Sense Multiple Access is similar to the way in which each person in a group of polite people waits to speak until the person speaking is finished.
Station 101 can transmit and receive using:
i. a first modulation scheme.
In contrast, stations 202-1 and 202-2 can transmit and receive using:
i. the first modulation scheme, and
ii. a second modulation scheme.
Because stations 102-1 and 102-2 can use a superset of the modulations schemes available to station 101, they are called “enhanced” stations. In contrast, station 101 is called a “legacy” station.
The modulation schemes available to a legacy station are called “legacy modulation schemes” and the modulation schemes available to an enhanced station are called “enhanced modulation schemes.”
Any two stations that need to communicate must do so in accordance with a modulation scheme that is available to both of them. Therefore, two enhanced stations can communicate with any of the enhanced modulation schemes, but any communication involving a legacy station must use a legacy modulation scheme.
Stations 102-1 and 102-2 communicate with each other using the second modulation scheme when possible because it enhances communication throughput in comparison to the first modulation scheme. One effect of using the second scheme, however, is that station 101 cannot detect when stations 102-1 and 102-2 are communicating (i.e., stations 102-1 and 102-2 are essentially invisible to station 101 when stations 102-1 and 102-2 are communicating using the second modulation scheme). This can cause station 101 to transmit when stations 102-1 and 102-2 are communicating, which causes all of the transmissions to be garbled.
To address this problem, a method called “transmission protection” is known in the prior art to prevent legacy stations from transmitting while transmissions using the second modulation scheme are in progress.
In accordance with transmission protection, an enhanced station that is about to transmit a frame using the second modulation scheme first transmits a short frame using the first modulation scheme. This short frame is detectable by the legacy stations in the area.
A duration field in the short frame contains a value that indicates how long the legacy terminals should refrain from transmitting, and the field is populated with a duration that is long enough to cover the length of time for transmissions of frames using the second modulation scheme. The duration information inside the Request-to-Send or Clear-to-Send frame activates a virtual carrier sense mechanism in the legacy stations, which will not transmit, as a result, during the protected, subsequent second transmission.
A mechanism is also known in the prior art for notifying all of the enhanced stations in the network when to use and when not to use transmission protection. In accordance with this mechanism, one enhanced station has the capability to activate and deactivate transmission protection in the other enhanced stations by signaling whether or not protection must be used.
If transmission protection is disabled and there is a heavy traffic load using the second modulation scheme, transmissions from legacy stations will repeatedly collide with transmissions using the second modulation scheme and there might be an unreasonable delay before an enhanced station notices that a legacy station is trying to transmit. As a result, there might be a delay in activating transmission protection in the enhanced terminals.
Therefore, the need exists for a technique for reasonably activating transmission protection in enhanced terminals.